Electric power demand varies greatly during the day. The consumption that generates the demand is at the highest in the day time when industrial activities are also at the highest level. The demand is reduced to its minimum during the night time when less power is needed for lighting and when other industrial activities are also at the lowest level. The wide variations of daily power demand is also influenced by low consumption during weekends when less power is needed due to reduced industrial or business activities. Seasonal effects such as high load for air conditioning during the hot summer seasons or high heating loads in cold winter months also have additional impacts on the levels of power peaks or power off-peaks. This wide fluctuation is well known and utility companies must cope with it by providing spare capacity on the power grid to accommodate higher demand periods, and by having equipment configuration such that power blocks or generating units are capable of being taken off line when demand drops.
Coal power plants and nuclear power plants, typically low cost fuel power plants, are relatively slow in capacity adjustment and load adaptation due to heavy equipment's inertia or safety constraints. For this reason, they are usually operated as base load plants to satisfy the core of the requirement.
When power demand increases, backup peak shaving gas turbines, operated on relatively costly natural gas, can be started to keep up with the demand. It is obvious that the backup equipment must be able to get on stream very quickly because the demand peaks can occur quite rapidly and usually, instantaneously. It is also clear that, mainly because of economic reasons, the backup equipment cost must be minimized since they are only needed for short durations and not permanently. Gas turbines for base load plants are usually equipped with combined cycle to maximize the overall cycle efficiency. Steam boiler and condensing steam turbines of the combined cycle are high cost items and require relatively long time, in a matter of several hours, to be fully on stream. Because of those shortcomings, peak shaving gas turbines are simple cycle, not equipped with combined cycle, to yield the lowest investment cost per kW installed. Therefore the efficiency of the peak shaving gas turbine must be compromised.
The fluctuations of the power demand can be smoothed out by providing an energy storage system: power is stored during the low demand periods and disbursed back to the grid during the high demand periods. A typical example of this setup is a hydraulic water pumping system: the surplus of power during the off-peak periods can be used to drive water pumps to send water from a low basin reservoir to a reservoir located at a higher elevation. When power demand increases, this water is returned to the low elevation reservoir by sending it to hydraulic turbines to generate supplemental electric power. The ramp-up is quite fast for this system. However, this setup is of course not applicable to most power plants since it requires an expensive infrastructure using high and low elevation reservoirs along with multiple large hydraulic turbines. In addition to the high global investment cost, the recovery, defined as the ratio of electricity output over the electricity input, is only in the range of about 60% due to the fact that the reservoirs are likely to be at remote locations such that transmission line losses can be quite high and the efficiencies of the pumps and hydraulic turbines are in the range of only about 70%. Therefore an efficient and economical process of storing energy is desirable to address the issue of power demand fluctuations.
More and more power generation plants are being built with combustion gas turbine technology. Because of environmental issues, coal based power plants with gasification technology (IGCC Integrated Gasification Combined Cycle) are being built or selected for several projects. In the regions of the world where natural gas is available at relatively low cost, combined cycle natural gas power plant for base load operation is the technology of choice. Gas turbine concept by itself is not very efficient since about 50% of the turbine's power is wasted to compress the air for the combustion and expansion. However, the gas turbine cycle efficiency is improved significantly by adding a steam combined cycle on the turbine's exhaust gas: the waste heat of the exhaust gas is used to heat and vaporized water to form high pressure steam which is then expanded in steam turbines to generate additional power. The combined cycle concept is widely used today in the power generation industry. However, because of the complexity and the high cost of the multiple pressure heat recovery steam generation system (HRSG) and the steam turbines, and the heavy infrastructure of the very large cooling tower for the steam condensing circuit, the steam combined cycle can only be justified economically for plants larger than about 50 MW or even 100 MW. Plant size can be smaller in case of cogeneration when clients are available to purchase steam produced by the facility and to partially pay for the cost of the steam system. Because of this economic constraint, many small plants are operated based on a simple cycle concept, i.e. no combined cycle, with significant penalty on the cycle efficiency. Gas turbine vendors are implementing several improvements to the gas turbine technology in order to reduce the impact of poor efficiency such as increasing pressure ratio thus reducing exhaust temperature, or improving turbine's blade heat resistance to accommodate higher inlet temperature or using recuperated gas turbine approach. However those changes only result in smaller incremental improvement to the process efficiency. Therefore another approach less costly than the steam combined cycle capable of improving the efficiency of the gas turbine power generation system is highly desirable especially for the small and medium size plant application.
When power demand increases, backup peak shaving gas turbines, operated on relatively costly natural gas, can be started to keep up with the demand. It is obvious that the backup equipment must be able to get on stream very quickly because the demand peaks can occur quite rapidly and usually, instantaneously. It is also clear that, mainly because of economic reasons, the backup equipment cost must be minimized since they are only needed for short durations and not permanently. Gas turbines for base load plants are usually equipped with combined cycle to maximize the overall cycle efficiency. Steam boiler and condensing steam turbines of the combined cycle are high cost items and require relatively long time, in a matter of several hours, to be fully on stream. Because of those shortcomings, peak shaving gas turbines are simple cycle, not equipped with combined cycle, to yield the lowest investment cost per kW installed. Therefore the efficiency of the peak shaving gas turbine must be compromised.
Atmospheric air is a potential candidate for the medium used for energy storage. For example, air can be compressed during off-peak periods to higher pressure and stored in large underground cavern created by solution mining. During peak load periods, pressurized air of the storage can be heated by combusting natural gas to high temperature then expanded in gas turbine for power recovery. The efficiency of the power recovery depends upon the type of compression used to compress the air: adiabatic, diabatic or isothermal. This concept is simple but, similar to the water pumping scheme, requires important capital expenditure for the infrastructure. Site locations in case of mining solution are usually very remote.
To minimize the storage size and the associated cost of compressed air system, air can be liquefied by cryogenic technique and stored economically in large quantity in conventional storage tank. This air, in liquid form, can be vaporized and transformed into gaseous form to restore the compressed air needed for power generation. This technique is promising because it facilitates the compress air energy storage approach without the high cost associated with the underground cavern at remote locations. A facility for air liquefaction can be easily deployed near the main users like large cities. The technology of air liquefier and cryogenic storage are very well known and can be implemented quickly and reliably. However, several technical issues must be resolved before this approach can be used economically.
An object of this invention is to provide a technique of using liquid air to store energy. Liquefaction of air requires energy input, the specific power required to liquefy the air is about 0.5 kWh/Nm3. The liquefaction power can be improved slightly at the expense of higher investment cost for the equipment. This energy input must be recovered efficiently in the vaporization step otherwise the overall process efficiency will suffer. Therefore it is desirable to provide an efficient process for liquid air vaporization.
Considering that the liquefaction is an energy intensive process, it is advantageous to avoid this liquefaction during the peak load periods where power cost is at the premium. Therefore liquefaction during off-peak periods, for example at night time, will maximize the cost effectiveness of the concept. Power consumption for equipment such as compressors in the vaporization step must be kept at a minimum.
One potential technique of reducing power consumption of equipment is to utilize the cold or refrigeration supplied by cryogenic liquid of the cold compression process. Cold compression reduces the power consumption of the compressor significantly because the inlet temperature of the compressor is at very low level, usually in the range of −180° C. to −60° C. However, the main penalty of the cold compression is that the heat generated by the compression, even quite low at cryogenic level, must be evacuated at that cryogenic temperature level such that the required refrigeration will adversely effect the overall power consumption. In case the source of refrigeration available for the heat removal is a low cost cryogenic liquid produced inexpensively during off-peaks then cold compression becomes quite attractive.
This invention relates to an improved technique of using liquid air as the energy storage medium. Liquid air produced and stored in off-peak periods can be restored to compressed air under high pressure by an efficient vaporization process assisted with cold compression technology. The compressed air is then heated and expanded in a compressed air combined cycle to generate additional power in peak periods and to improve the efficiency of the gas turbine without a costly steam combined cycle.
The use of this invention can extend the concept of combined cycle to medium and small power gas turbine power generating units without the high cost and slow response of the traditional steam turbine combined cycle.